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pgmcrater - create cratered terrain by fractal forgery
pgmcrater
[-number n] [-height|-ysize s] [-width|-xsize s] [-gamma g]
All options can be
abbreviated to their shortest unique prefix.
pgmcrater creates
a PGM image which mimics cratered terrain. The PGM image is created by
simulating the impact of a given number of craters with random position
and size, then rendering the resulting terrain elevations based on a light
source shining from one side of the screen. The size distribution of the
craters is based on a power law which results in many more small craters
than large ones. The number of craters of a given size varies as the reciprocal
of the area as described on pages 31 and 32 of Peitgen and Saupe[1]; cratered
bodies in the Solar System are observed to obey this relationship. The
formula used to obtain crater radii governed by this law from a uniformly
distributed pseudorandom sequence was developed by Rudy Rucker.
High resolution
images with large numbers of craters often benefit from being piped through
pnmsmooth. The averaging performed by this process eliminates some of the
jagged pixels and lends a mellow ``telescopic image'' feel to the overall picture.
pgmcrater simulates only small craters, which are hemispherical in shape
(regardless of the incidence angle of the impacting body, as long as the
velocity is sufficiently high). Large craters, such as Copernicus and Tycho
on the Moon, have a ``walled plain'' shape with a cross-section more like:
/\ /\
_____/ \____________/\____________/ \_____
Larger craters should really use this profile, including the central peak,
and totally obliterate the pre-existing terrain.
- -number n
- Causes
n craters to be generated. If no -number specification is given, 50000 craters
will be generated. Don't expect to see them all! For every large crater
there are many, many more tiny ones which tend simply to erode the landscape.
In general, the more craters you specify the more realistic the result;
ideally you want the entire terrain to have been extensively turned over
again and again by cratering. High resolution images containing five to
ten million craters are stunning but take quite a while to create.
- -height
height
- Sets the height of the generated image to height pixels. The default
height is 256 pixels.
- -width width
- Sets the width of the generated image
to width pixels. The default width is 256 pixels.
- -xsize width
- Sets the width
of the generated image to width pixels. The default width is 256 pixels.
- -ysize height
- Sets the height of the generated image to height pixels. The
default height is 256 pixels.
- -gamma factor
- The specified factor is used
to gamma adjust the image in the same manner as performed by pnmgamma. The
default value is 1.0, which results in a medium contrast image. Values larger
than 1 lighten the image and reduce contrast, while values less than 1
darken the image, increasing contrast.
Note that this is separate from
the gamma correction that is part of the definition of the PGM format.
The image pnmgamma generates is a genuine, gamma-corrected PGM image in
any case. This option simply changes the contrast and may compensate for
a display device that does not correctly render PGM images.
The
-gamma option isn't really necessary since you can achieve the same effect
by piping the output from pgmcrater through pnmgamma. However, pgmcrater
performs an internal gamma map anyway in the process of rendering the elevation
array into the PGM format, so there's no additional overhead in allowing
an additional gamma adjustment.
Real craters have two distinct morphologies.
pgm(5)
, pnmgamma(1)
, pnmsmooth(1)
- Peitgen, H.-O., and Saupe, D. eds.,
The Science Of Fractal Images, New York: Springer Verlag, 1988.
John Walker
Autodesk SA
Avenue des Champs-Montants 14b
CH-2074 MARIN
Suisse/Schweiz/Svizzera/Svizra/Switzerland
- Usenet:kelvin@Autodesk.comFax:038/33 88 15Voice:038/33 76 33
Permission
to use, copy, modify, and distribute this software and its documentation
for any purpose and without fee is hereby granted, without any conditions
or restrictions. This software is provided ``as is'' without express or implied
warranty.
PLUGWARE! If you like this kind of stuff, you may also enjoy ``James
Gleick's Chaos--The Software'' for MS-DOS, available for $59.95 from your local
software store or directly from Autodesk, Inc., Attn: Science Series, 2320
Marinship Way, Sausalito, CA 94965, USA. Telephone: (800) 688-2344 toll-free
or, outside the U.S. (415) 332-2344 Ext 4886. Fax: (415) 289-4718. ``Chaos--The
Software'' includes a more comprehensive fractal forgery generator which
creates three-dimensional landscapes as well as clouds and planets, plus
five more modules which explore other aspects of Chaos. The user guide
of more than 200 pages includes an introduction by James Gleick and detailed
explanations by Rudy Rucker of the mathematics and algorithms used by each
program.
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